In the modern building, wiring holes (not shown) are reserved on the walls of every house for wiring telecommunication lines. As shown in FIG. 1, a wiring box 1 is mounted in a wiring hole. In addition to be wired to an outdoor telecommunication cable for the purpose of external communication, the wiring box 1, which is mounted everywhere in the indoor space, is also wired to an indoor telecommunication cable so as to interlink the indoor extensions. However, in practice, the existing wiring box 1 is only capable of being wired to one or two telecommunication line(s). It provides with no function related to the extension line and cannot be wired to multiple telecommunication lines. Furthermore, the connection between the telecommunication lines and every core wire are insulated by tape. It results in inconvenience since the cover 11 is closed. Accordingly, when the telecommunication line is failed, the maintenance worker must detach the whole cover 11 of the wiring box 1 first, and then pull out the telecommunication cable for peeling off the tape, which winds the outside of the telecommunication cable, before repairing the telecommunication line. Consequently, the entire repair process is very complicated and time-consuming.